1. Field of the Invention
The present invention relates to the use of lasers to cut metals, and more specifically, it relates to the use of ultrashort laser pulses for machining metal.
2. Description of Related Art
Conventional mechanical lathes and machine tools (e.g., slitting saws) are effective for cutting metals down to approximately 100 microns kerf width at depths on the order of 1 millimeter (aspect ratio  less than 10:1). Below this level, electron beam or laser tools are typically used for cutting or high precision machining (sculpting, drilling). Both electron beam and existing industrial laser technology remove material by a conventional thermal process where the material to be removed is heated to the melting or boiling point. Laser processing by molecular dissociation in organic (and some inorganic) materials can be achieved with excimer lasers but this photodissociation mechanism is not applicable to metals.
The basic interaction in localized thermal processing, as is achieved with electron beam or current state of the art lasers, is the deposition of energy from the incident beam in the material of interest in the form of heat (lattice vibrations). Cutting efficacy and quality may differ strongly between metals dependent upon the thermomechanical properties of the metal. Laser absorption is also dependent upon the optical properties of the metal of interest. The laser energy that is absorbed results in a temperature increase at and near the absorption site. As the temperature increases to the melting or boiling point, material is removed by conventional melting or vaporization. Depending on the pulse duration of the laser, the temperature rise in the irradiated zone may be very fast resulting in thermal ablation and shock. The irradiated zone may be vaporized or simply ablate off due to the fact that the local thermal stress has become larger than the yield strength of the material (thermal shock). In all these cases, where material is removed via a thermal mechanism, there is an impact on the material surrounding the site where material has been removed. The surrounding material will have experienced a large temperature excursion or shock often resulting in significant change to the material properties. These changes may range from a change in grain structure to an actual change in composition. Such compositional changes include oxidation (if cut in air or, in the case of alloys, changes in composition of the alloy. This affected zone may range from a few microns to several millimeters depending on the thermomechanical properties of the metal, laser pulse duration and other factors (e.g., active cooling). In many applications, the presence of the heat or shock affected zone may be severely limiting since the material properties of this zone may be quite different than that of the bulk. Furthermore, devices with features on the order of a few tens of microns cannot tolerate the thermal stress induced in the material during the machining process. Even the slightest thermal stress or shock can destroy the feature of interest.
Another limitation of conventional laser or electron beam processing in high precision applications is the presence of redeposited or resolidified material. As mentioned previously, cutting or drilling occurs by either melting or vaporizing the material of interest. The surface adjacent to the removed area will have experienced significant thermal loading often resulting in melting. This melting can be accompanied by flow prior to solidification as shown in FIG. 1A. This can result in the deposition of slag surrounding the kerf. In many high precision applications, the presence of slag is unacceptable. In the cases where the deposition of conventinal slag can be prevented, redeposition of vaporized material on the walls or upper surface of the kerf is common. This condensate often reduces the quality of the cut and decreases the cutting efficiency since the beam must again remove this condensate before interacting with the bulk material underneath. FIG. 1A shows a top view of stainless steel cut with a conventional infrared (1053 nm) laser operating at a pulse duration  greater than 1 nsec. The presence of resolidified molten material (slag) and poor single pass cut quality indicative of laser cutting by conventional methods is readily apparent.
Many of these limitations can be reduced by the use of secondary techniques to aid the cutting process. The most common of these are active cooling of the material of interest either during or immediately following the laser pulse, and the use of high pressure gas jets to remove vaporized or molten material from the vicinity of the cut to prevent redeposition. These techniques can be effective at improving the kerf at the cost of a significant increase in system complexity and often a decrease in cutting efficiency.
It is an object of the present invention to provide a method for laser cutting/machining of metals and alloys which achieves high machining speed with extreme precision, negligible heat affected zone, and no modification to the material surrounding the kerf.
The method involves the use of very short (10 femtoseconds to approximately 100 picoseconds) laser pulses delivered at high repetition rate (0.1 to over 100 kHz). Although the absorption mechanism for the laser energy is the same as in the case of long pulse lasers, the short ( less than 100 psec) duration offers a simple and striking advantage. By adjusting the pulse duration such that the thermal penetration depth during the pulse, Lth=2xcex1xcexa3(xcex1=k/xcfx81cp is the thermal diffusivity, k is the thermal conductivity, xcfx81 is the density, cp is the heat capacity and xcfx84 is the duration of the laser pulse] is less than one micron, very small amounts of material (0.01-1 micron) can be removed per laser pulse with extremely small transport of energy either by shock or thermal conduction away from the volume of interest. This offers extremely high precision machining with a negligible (submicron) heat or shock effected zone. For example, type 304 stainless steel exhibits a thermal penetration depth of only 1.5 nm for a 100 femtosecond pulse compared to an optical penetration depth of approximately 5 nm. In this case, the electric field of the laser penetrates more deeply into the steel than the thermal wave during the pulse. Hence, the depth of material removal is determined solely by the intensity and wavelength of the laser, and the absorption and heat capacity of the metalxe2x80x94the effects of heat conduction and thermal shock are eliminated.
The lack of significant energy deposition beyond the volume of interest achieved by using these ultrashort pulses enables the use of high repetition (0.1-100 kHz) lasers without the need for external cooling of the part being machined. Even though only a very small depth of material is removed per pulse, the high repetition rate enables extremely high cut rates (beyond 1 mm depth per second).
Cut quality and cut efficiency with these ultrashort pulses can be significantly higher than that achievable for conventional long pulse lasers. This follows from two critical features: 1) there is little loss of energy away from the region of interest since thermal conduction during the pulse is negligible and 2) there is no vaporization or transport of material during the pulse. The second of these features may require additional explanation. During the pulse, there is insufficient time for hydrodynamic expansion of the vaporized material. As a result, the laser pulse encounters the solid surface for the duration of the pulse, depositing energy into the solid density material and raising a depth to a temperature far beyond the boiling point (typically to temperatures above the ionization point). After the pulse is over, the depth which has been raised above the boiling point leaves the surface with an expansion velocity determined by the initial temperature. Typical temperatures in the expanding plasma are between 1 and 100 eV and are determined by the product of the incident laser irradiance, I(W/cm2) and square of the laser wavelength, xcex2(xcexcm). The high plasma temperature insures that the vaporized material will be completely removed from the kerf without redeposition on the walls. This material is removed before the arrival of the next laser pulse 0.01 to 10 milliseconds later. For example, an expanding vapor with even a low expansion velocity of 105 cm/sec will be 1 meter away from the surface before the arrival of the next pulse if operating at a 1 kilohertz repetition rate. With conventional nanosecond or microsecond lasers, the vapor will evolve during the laser pulse. This reduces the coupling of the laser light to the solid surface since the incident laser light will be scattered and absorbed by the vapor. This problem is completely overcome by the use of the very short pulses of the present invention.
High precision machining of metals is an extremely large industry world-wide. This industry continues to grow with new applications particularly in semiconductor processing and display technology appearing frequently. Conventional mechanical lathes and machine tools are effective for cutting applications down to approximately 100 microns kerf width. Below this level, electron beam or laser tools are typically used for cutting or high precision machining (sculpting, drilling). Both electron beam and existing industrial laser technology remove material by a conventional thermal process where the material to be removed is heated to the melting or boiling point. The temperature of the surrounding material is determined by standard heat conduction from the region of interest. While small scale features ( less than 100 microns) are readily achieved, they are often surrounded by resolidified material (slag) and by a significant heat affected or shock zone often requiring post processing (e.g., annealing, electro-polishing, etc.). This heat affected zone alters the properties of the material in the vicinity of the machined surface, often resulting in reduced material strength or modification of the composition of the material in the case of alloys. The present invention converts the region to be removed from the solid-state to the plasma state so quickly that there is insufficient time for significant heat transfer beyond the depth of material removed. This results in the ability to perform extremely high precision machining of metals or alloys with essentially no heat affected zone and eliminates the need for cooling of the part during the machining process. Hydrodynamic expansion of the plasma eliminates the need for any ancillary source to aid in material removal such as gas flow.
This invention enables, but is not limited to, applications requiring high precision ( less than 100 microns) or remote machining (cutting, drilling or sculpting) with no change in the remaining material (grain structure, alloy composition, materials strength, etc.). Specific examples include the production of microgears and valves, insertable medical devices and disk patterning.